<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>

<body>
  <style>
    *,html,body{
      margin:0;
      padding:0;
      box-sizing: border-box;
    }
    #wrapper {
      position: absolute;

      width: 100%;
      height: 100%;
      background-color: #f00;
    }

    #box {
      width: 80%;
      height: 80%;
      background-color: orange;
    }

    #el {
      width: 100%;
      height: 60%;
      background-color: yellowgreen;
    }

    #children {
      width: 40%;
      height: 80%;
      background-color: gray;
    }
  </style>

  <div id="wrapper">
    <div id="box">
      <div id="el">
        <div id="children"></div>
      </div>
    </div>
  </div>

</body>

</html>